Electron gun



Oct. 29, 1957 G. R. BREWER ELECTRON GUN 2 Sheets-Sheet 1 Filed Dec. 9,1954 E El T/ Oct. 29, 1957 G. R. BREWER 7 ELECTRON GUN Filed Dec. 9.1954 2 Sheets-Sheet 2 Wmwz.

lira/Vii United States Patent ELECTRON GUN George R. Brewer, PalosVerdes Estates, Califl, assignor to Hughes Aircraft Company, CulverCity, Calif a corporation of Delaware Application December 9, 1954,Serial No. 474,07 8

4 Claims. (Cl. 315-) This invention relates to electron tubes anl moreparticularly to an electron gun for producing an electron stream havinga substantially uniform diameter over its length.

Electron guns which are employed to produce relatively high currentelectron streams generally comprise a cathode having an emission surfacewhich is concave with respect to the electron stream, a frusto-conicalfocusing electrode disposed concentrically about the cathode, and adish-shaped anode having an apertured central surface concave withrespect to and spaced from the cathode emission surface. All of the gunelectrodes are properly shaped and maintained at appropriate potentialsto cause substantially all of the electrons which emanate from thecathode to be focussed toward the same point along a predeterminedelectron stream path. The gun electrodes are thus employed to develop asolid cylindrical electron stream having a uniform current density overits cross section. When the gun actually accomplishes this result, anelectron stream thus produced is said to be well collimated.

It may be particularly desirable to produce collimation of electrons inan electron stream which is magnetically focused in a manner well knownin the art as Brillouin flow. This is true because the Brillouin type offlow allows a given stream to be focussed or confined with a magneticfield of a minimum strength. However, unless certain precautions aretaken in the design of the gun and magnetic field system, electronstream diameter variations invariably occur in Brillouin flow. Thesevariations physically manifest themselves as a plurality of rippleswhich are disposed at periodic stationary intervals along the streamcausing the stream to expand and contract. When such a stream isemployed in a microwave tube, such as a traveling-wave tube, theeificiency of the tube is generally reduced by the stream diametervariations. These ripples can also cause a number of other deleteriouseffects on traveling-wave tube performance. in such a case it istherefore definitely desirable to minimize these variations by producinga collimated electron flow.

At present it is frequently the practice to design the severalelectrodes of a converging beam gun as though the anode aperture throughwhich the beam passes were not in existence. A focusing error is thusproduced which impairs a collimated electron flow. This focusing errorresults in a large part from the distortion of the electric field lines,imparting undesirable transverse motion to the electrons.

An object of the invention is, therefore, to provide an improvedelectron gun for a traveling-wave tube.

Another object of the invention is to provide an electron gun fordeveloping a substantially collimated flow of electrons.

in accordance with the invention as second anode having a cylindricalinternal surface is disposed adjacent to and partly within thedish-shaped anode of an otherwise more or less conventional convergingbeam gun. The

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electric field contiguous to the gun cathode may thus be modified insuch a way as to be more uniform and thus to improve collimation whenthe second anode is maintained at a potential somewhat greater than thatof the dish-shaped anode.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description taken in connection with the accompanyingdrawings, in which several embodiments of the invention are illustratedby way of example. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only, andare not intended as a definition of the limits of the invention.

Figs. 1 and 2 are sectional views of different embodiments of anelectron gun constructed in accordance with the invention;

Figs. 3 and 4 are schematic views of an electric field distributionwhich may exist contiguous to the cathode of the gun shown in Figs. 1and 2; and

Figs. 5 and 6 are diagrammatic views of an electrolytic tank which maybe employed in the designing of the gun of the present invention.

Referring to Fig. 1, an electron gun 10, which may, for example, have aperveance of 2x10 or higher, is shown comprising a cathode 12 which isprovided with a filament 14, a focusing electrode 16, a dish-shapedanode 13, and a second hollow cylindrical anode 20.

Cathode 12 is itself a hollow cylinder having an enclosed end portion 22concave with respect to its electron stream and which serves as anelectron emissive surface. The emissive surface may be, for example,spherical. Filament 14, which has a helical shape,is surrounded by aconcave dish-shaped block of a refractory material 24. Block 24 andfilament 14 are thus situated concentrically within the cylinder of thecathode 12 adjacent the end portion 22.

Focusing electrode 16 is disposed concentrically about the cylinder ofthe cathode 12 having a frusto-conical internal focusing surface 26.Focusing electrode 16 also has an aperture 28 in a disc-shaped endportion 29 through which a concave sheet 30 forming the central portionof dish-shaped anode 18 extends. Dishshaped anode 18 is likewiseprovided with an aperture 32 through which electron fiow is produced.

Cylindrical anode 20 is disposed partially within the concave sheet 30in order that an improved collimation of electrons along a predeterminedpath, e. g., a path 34, may be produced. Cylindrical anode 20 isprovided with an encompassing supporting disc 31 with which cylindricalanode 20 may be mounted in an electron tube.

Filament 14 is heated by a filament battery 36 which is connectedthereacross. Cathode 12 and the negative terminal of battery 36 aremaintained at ground potential by a suitable connection thereto.

Focusing electrode is may be maintained a few volts negative withrespect to ground by a focusing electrode source of potential 38.Depending upon the position and exact shape of focusing surface 26,focusing electrode 16 may be maintained negative or at zero potentialwith respect to the potential of cathode 12.

Dish-shaped anode'ls is maintained at a relatively large positivepotential with respect to that of focusing electrode 16 by means of afirst anode source of potential 40. Cylindrical anode 20 is maintainedat a still larger positive potential by a second anode source ofpotential 42.

Through empirical studies of electric field patterns in an electrolytictank, which method of examination is well known in the art, it has beenfound that best collimated electron flow is produced in the range ofdepending on the design the electrode 20 and its disposition withrespect to anode 32, where V2 is the voltage of cylindrical anode 20 andV1 is the voltage of dishshaped anode 18. Some improvement in electroncollimation can be obtained when V2 is simply larger than Vi; however,if V2 is more than twice as large, the improvement may be negligible,and the performance may even be impaired.

The electrodes of the gun 10, excluding the second anode 20, may bedesigned according to the procedure related in U. S. Patent No.2,268,165, granted December 30, 1941, to C. V. Parker et al. The cathode12, the focusing electrode 16, and the first anode 18 may also bemaintained at appropriate relative potentials as set out in the Parkerpatent.

The principal reason for using the cylindrical anode 20 arises out ofthe fact that for a relatively high perveance gun the size of the firstanode aperture diameter is of the same order of magnitude as thedistance from the first anode to the cathode. This generally decreasesthe focusing capabilities of the dish-shaped anode 18 by causing adistortion of the electric equipotential lines from their desiredspherical shape. However, this effect is essentially negligible inrelatively low perveance guns.

The gun is shown again in Fig. 2 with both a differently shaped focusingelectrode 120 and first anode 122. Focusing electrode 120 performs thesame function as focusing electrode 16; however, focusing electrode 120consists simply of a frusto-conical metallic sheet. The structure offirst anode 122 differs from that of dishshaped anode 18 in that theinterior of the dish-shaped outer wall of the first anode 122 ispartially filled with metal in order to improve the performance of thegun 10 and shaped to accommodate a portion of cylindrical anode 20.

The schematic diagrams of Figs. 3 and 4 illustrate in particular how thecylindrical anode 20 may be emloyed to improve the collimation of streamelectrons when maintained with a potential range set out in theinequalities of the expression (1). Cathode 12, focusing electrode 120and first anode 122 are shown in both Figs. 3 and 4 whereas cylindricalanode 20 is only shown in Fig. 4. A plurality of dashed lines 33 in Fig.3 and 44 in Fig. 4 represent equipotential lines which would be producedby the electrodes shown in the respective Figs. 3 and 4.

In order to produce a collimation of stream electrons, it is necessaryto direct the electrons emitted at the end portion 22 of cathode 12toward a common focusing point, viz. points 46 in Figs. 3 and 4.However, in Fig. 3 where cylindrical anode 20 is not employed, thetransverse forces resulting from the distortion of the equipotentiallines 33 will cause electrons at the outer edge of the stream to bedirected toward the focal point 46 nearer the cathode 12 than a focalpoint 47 to which the electrons at the center of the stream will bedirected. This is true because the equipotential surfaces represented bythe dashed lines are not concentric; giving rise to transversedefocusing fields which act upon the stream electrons. The density ofelectron emission in Fig. 3 likewise varies with the radius of theelectron stream, whereby collimation may be exceedingly poor. In extremecases a large number of the electrons may be caused to strike the anode.However in Fig. 4 where cylindrical anode 20 is employed, all of theequipotential lines 44 are substantially parallel and good collimationis produced. The magnitude of ripples in Brillouin flow may thereby bereduced and the efficiency of microwave tubes, such as, for examp traelin -w v tubes, ma be thu increase n o e to bta a p ope ly ped d spa edse ond anode and to obtain the optimum potential at which it should bemaintained, an electrolytic tank design procedure may be mosteffectively employed. Such a tank is illustrated in Figs. 5 and 6 and isfilled with water 151 up to a water level 153. A first conductive sheet152 in the tank represents one-half of the cathode surface 22. A secondconductive sheet 154 illustrated only in Fig. 6 represents focusingelectrode 120. A third conductive sheet 156 represents first anode 122,and a fourth conductive sheet 158 is employed to represent the structureof cylindrical anode 20. Suitable potentials are applied to sheets 152,154, and 156, and the potential applied to sheet 158 is varied as theelectric field pattern is detected by a probe disposed in the water 151.

In arriving at a correct design for the various electrodes of the gun,one must vary two parameters, the position and shape of the focusingelectrode so as to achieve the desired potential distribution along thebeam boundary and the shape, position, and potential of the second anode158 relative to' the anode in such a Way that the electric gradientadjacent to the surface of the cathode is uniform over the cathodesurface. These desired shapes can be obtained as follows: In theelectrolytic tank 150 the required boundary conditions at the edge ofthe beam are: (a) the electric gradient normal to the beam boundary mustbe zero, and (b) the potential must be continuous across this boundary.These conditions are fulfilled by placing a dielectric strip 157 alongthe desired beam boundary with a seriesof thin spaced metallic strips161 which can be held at the desired potentials by external circuitryindicated by 159. The presence of the dielectric strip 157 insures thatno current flows normal to the beam boundary in the tank, thus formingthe analog of the zero normal field condition. In addition, the metallicstrips 161 can be adjusted in potential to provide a potentialdistribution along the inner surface which is identical to that producedon the outside of the dielectric strip 157 by the focusing electrode120, thus satisfying the second boundary condition. The potential alongthe edge of the beam should follow that found by Langmiur and Blodgett,Phys. Rev. 24, page 49 (1924), which is where o: is represented by theseries of Langmiur and -a is the value of a at the anode, or any otherdesired and suitable distribution law. The position and shape of theconductive sheet 154 may then be determined in the manner indicated inFig. 6 where 163 indicates a metering device for ascertaining thepotential distribution along the metallic strips 161.

The shape and potential of electrode 15% is adjusted while measuring theelectric gradient normal to the cathode at several points therealonguntil this gradient is substantially uniform. Under this condition itcan reasonably be assumed that the emission density will be uniform, theequipotential lines being essentially concentric with the cathodeemission surface and the focusing properties of the gun being thatdesired. The dependence of emission current density on the normalpotential gradient contiguous to the cathode emission surface may beobserved by reviewing the emission equation. For example, using planarequations a i the fir t anod l ag a L s e fir seed erasin is the.gradient immediately adjacent the cathode in the absence of space chargethen E0 2 Jo K(, (4) Thus if E0 is uniform over the cathodesurface,.then Jo will also be uniform over the cathode surface.

It is then evident that a converging beam electron gun may beconstructed which will give a collimated electron stream with a minimumdisturbance of rippling in an axial magnetic field. This is madepossible by the uniform emission current density provided and by thesymmetrical focusing which may be obtained in practicing the presentinvention. e

What is claimed is:

1. An electron gun for producing a substantially collimated flow'ofelectrons, said electron gun comprising a thermionic cathode having aconcave emission surface with respect to emitted electrons, a focusingelectrode disposed about said cathode, an anode having a centraldish-shaped surface disposed partially within said focusing electrode,said central portion of said anode being concave toward said cathode,and a second anode having 1 one end disposed partially within saiddish-shaped anode,

said second anode having a cylindrical internal surface extendingthroughout its length to permit an electron flow therethrough, wherebyequipotential surfaces with curvatures substantially concentric to theemission surface of said cathode may be produced by the application ofappropriate voltages to said anodes, said cathode, electrode, and anodesbeing concentrically aligned with each other.

2. An electron gun for developing a substantially collimated flow ofelectrons along a predetermined path, said electron gun comprising athermionic cathode having a substantially concave spherical electronemissive surface with respect to emitted electrons, a focusing electrodedisposed about said cathode having a substantially frustoconicalinternal focusing surface converging inwardly toward said cathode, adish-shaped anode having an apertured central surface disposed concavetoward said cathode and disposed partially within the frusto-conicalsurface of said focusing electrode, a hollow cylindrical anode disposedabout said path having one end disposed partially within the centralportion of said dish-shaped anode, means for maintaining saiddish-shaped anode at a first predetermined potential with respect tosaid cathode, and means for maintaining said cylindrical anode at asecond predetermined potential with respect to said cathode larger thansaid first predetermined potential said cathode, electrode, and anodesbeing concentrically aligned along said predetermined path.

3; An electron gun for producing a substantially collimated flow ofelectrons along a predetermined path, said electron gun comprising athermionic cathode having an emission surface concave with respect toemitted electrons, a focusing electrode disposed about said cathode, adish-shaped anode having a central dish-shaped surface disposedpartially within said focusing electrode, said central portion of saiddish-shaped anode being concave toward said cathode, a second anodehaving one end disposed partially within said dish-shaped anode, saidsecond anode having a cylindrical internal surface extending throughoutits length to permit electron flow therethrough, and means formaintaining said second anode at a potential. greater than that of saiddish-shaped anode with respect to said cathode and less than twice thepotential of said dish-shaped anode with respect to said cathode wherebyequipotential surfaces substantially parallel to the emission surface ofsaid cathode may be produced said cathode, electrode, and anodes beingconcentrically aligned along said predetermined path.

4. An electron gun for developing a substantially collimated flow ofelectrons along a predetermined path, said electron gun comprising athermionic cathode having a substantially spherical electron emissivcsurface concave with respect to emitted electrons, a focusing electrodedisposed about said cathode having a substantially frustoconicalinternal focusing surface converging inwardly toward said cathode, adish-shaped anode having an apertured central surface concave towardsaid cathode disposed partially within the frusto-conical surface ofsaid focusing electrode, a hollow cylindrical anode disposed about saidpath having one end disposed partially within the central portion ofsaid dish-shaped anode, and means for maintaining said cylindrical anodeat a predetermined potential from 1.1 to 1.5 times the potential of saiddish-shaped anode with respect to said cathode.

References Cited in the file of this patent UNITED STATES PATENTS SzeghoOct. 30,

